When setting a cement plug in the NCS it has be done accordingly to the NORSOK
regulations[7] which this subchapter is based on. The abbreviation NORSOK is Norwegian and can be translated directly to “Norwegian offshore sector competitiveness”. The NORSOK standards are developed to ensure safety first of all, but also contribute to companies in how to do an efficient and correct job. Without these standards companies could make wrong judgments and base their choices for money purpose only, which will at some point affect the safety negatively both for the environment and human life.
By adding the NORSOK standards, the Norwegian petroleum industry set up a common set of
“game rules” to follow. The Petroleum Safety Authorities (PSA) uses these standards to regulate and manage the activities on the NCS, and can give sanctions if these regulations are not met by the companies.
There are several NORSOK standards, but P&A of a well is regulated in the D-010 standard.
The D-010 handles “well integrity in drilling and well operations”, and chapter 9 handles abandonment activities [7]. When P&A is performed on a well, these regulations are
important to follow, and the standard describes properties required by plug, placement, length and integrity of plug, and also how to verify the plug.
Both for permanent plugging and temporary plugging the same regulations applies in terms of barrier properties and placement. The only difference is that it should be a possibility to re-open the well safely on a temporary plugging, which means that the plugs either should be retrievable or drillable in a temporary situation. There is some cases where special rules apply, and that is when a well needs to be suspended. For example when intervention is needed the well can be suspended for a short period of time. Then it could be accepted to use the fluid column as a barrier, which is not accepted in the other above mentioned cases.
A temporary plugging could be performed if it is possible that the well should be reopened at some later time. A temporary plugging can be done because of the profitability of the well, lack of technology or lack of knowledge to continue production from the given well. No matter what the reason for a temporary abandonment is, the risk of leakage should always be assessed when abandoning a well. Based on the assessment, it should be decided by the PSA
if the well should be a well wither with or without monitoring. With monitoring the well can be plugged temporary for as long as needed, but requires continuously monitoring and routinely testing. Without monitoring, the well can only be plugged for a maximum of 3 years, and a program for frequently visual (look for bubbles) observation should be established.
A permanent well barrier (a cement plug for example) should according to NORSOK D-010 [7] have the following characteristics:
a) Provide long term integrity (eternal perspective);
b) Impermeable;
c) Non-shrinking;
d) Able to withstand mechanical loads/impact;
e) Resistant to chemicals/ substances (H2S, CO2 and hydrocarbons);
f) Ensure bonding to steel;
g) Not harmful to the steel tubular integrity.
There are also length, placement and number of barriers regulations described by NORSOK.
In a well it is required two barriers for each permeable zone that either has the possibility of flow to surface, or zones containing hydrocarbons. If a permeable zone contains water with no potential of flow to surface, the zone can be isolated with only one barrier. This means when isolating two reservoirs in a well, the first reservoir (from top) has to be isolated with a set of barriers (primary and secondary). The bottom reservoir also has to be isolated with a set of barriers, but it can use the first reservoirs primary barrier as its secondary barrier. It is though required to have a cross-flow barrier between the reservoirs to ensure no flow between them, so at least one barrier should separate the zones. This way this example well would have three barriers, with one of them working as a common barrier for both reservoirs.
The placement of the barriers is very strict and crucial to ensure well integrity. If a buildup in pressure takes place from the reservoir below the primary barrier, the formation (if it is an open hole section) or the casing should be able to withstand the pressure. This is necessary to prevent leakage of hydrocarbons. To make sure that the buildup pressure in a well in a worst case scenario does not exceed the formation or casing limits, it is important to set the barrier at the right depth.
If it is an open hole section that is being sealed off, the fracture gradient of a formation (from logging) can tell how much pressure the formation can handle at each given depth without fracturing. In the case of a cased hole, the burst pressure rating of the casing can also determine the setting depth of the barrier.
If the primary barrier breaks and hydrocarbons (or water) flows through, the secondary barrier needs to be able to withstand the same pressure buildup as the primary barrier, which means that the setting depth of the barriers needs to be designed for the secondary barrier. There are a couple of ways to find the setting depth of the barrier, both graphically and by calculations.
Graphically the setting depth can be found by use of the pore and fracture plot of the well.
The minimum setting depth of the barrier needs to be set at a height where the formation can withstand the pressures from hydrocarbons in the well without fracturing. By knowing the pore pressure at the reservoir depth, this is also the shut-in pressure (pressure below cement plug) that can be expected. When plugging the well this pressure could build up from below minus the pressure from the hydrostatic column of the reservoir fluid up to the plug height.
The pressure found at per example at shut in at 2000m if the reservoir is at 2400m would be the pressure at reservoir (Pres) minus the hydrostatic pressure from reservoir fluid column of 400 meters.
(Eq.1) (Eq.2)
This pressure needs to be equal or less than the fracture pressure at the given depth to qualify for a good setting depth. If the reservoir pressure is per example 250 bars (250*105Pa), the shut in pressure at 2000 meters with reservoir fluid at 450kg/m3 can be found to be from Eq.1:
250*105 – [450*9.81*(2400-2000)] = 232.342*105Pa = 232.342 Bar.
This setting depth can easily be found graphically by use of the pressure gradient of the reservoir fluid. This gradient creates a declining line in a pressure/depth plot, where it is possible to follow the line from reservoir pore pressure to crossing of the fracture pressure (hence Fig.4). Where the two lines meet will be the minimum setting depth of the secondary
well barrier. The primary barrier needs to be set below the secondary to ensure that both barrier plugs fulfill the requirements of integrity.
The pressure gradient of the fluid is always based on “worst case scenario”, which means that if there is gas in the reservoir, the gas density will be the base for the calculations as the shut in pressure will end up higher (worst case) with lower density fluid in the hydrostatic column below cement.
Figure 4: Calculation of plug setting depth
When the minimum depth of placement is calculated, it is also required that the barriers are set in an impermeable formation. Impermeable formation like shale prevents flow to be able to flow around the barriers through the formation. The purpose of setting the well barriers is to re-create the impermeable formation which prevented the oil from migrating in the first place, before it was drilled through. When creating a barrier just above the reservoir it is practically re-establishing of the cap rock. It is therefore important to have adequate length of
cement used in plugging to make sure the barrier keeps its integrity. Described in NORSOK [7] the required length of barriers are as Table 1 displays:
Table 1: NORSOK Length regulations [7]
Open hole cement
In this thesis, the open hole cement plug will be like the one in Fig.5, and will be according to the length requirements from the regulations in the above Table 1.
When plugging a well, there is also a need of an environmental plug in the top of the well (open hole to surface plug) as earlier mentioned. This is a tertiary plug, but cannot be used as a primary or secondary barrier to withstand pressure. This is basically a plug to more or less prevent items falling into the well as displayed in Fig.5.
Figure 5: Example of barriers open hole [7]